1. Field of the Invention
The present invention relates to systems for soil-less cultivation of plants in water and particularly to systems for growing plants on a buoyant structure which can float on an aqueous nutrient solution.
2. Description of the Prior Art
A commercially feasible system for hydroponic cultivation of plants, particularly when incorporated in a comprehensive system of controlled environment agriculture, must meet a number of requirements to be competitive with conventional growing methods. Because of the high capital investment and operating costs per unit of growing area associated with such systems, optimum utilization of available space for the highest possible density growing is essential. The concept of variable plant spacing--allowing each plant, as it develops, only as much space as it actually needs at each particular stage of growth--is one of the most important elements of such high density growing, particularly in the case of fast growing crops of relatively low unit value such as lettuce. Hydroponic techniques have now been developed that permit plants to be grown successfully in individual seed blocks, without the use of a continuous rooting medium of any kind, thus making possible free respacing of the growing plants as they mature. Such respacing tends, however, to be labor intensive and difficult to regulate precisely. It also leads to problems in providing adequate support for individual plants and in maintaining the light-tight shielding of nutrient solutions around the plants that is desirable for control of algae in hydroponic growing systems.
In addition, liquid nutrient solution supplied to the individual seed blocks must be replenished, either continuously or periodically. The resulting flow tends to shift the growing blocks from their originally set positions within the troughs or beds of the hydroponic system. It also is difficult to regulate the liquid nutrient depth such that plant roots are kept moist but the stems are not flooded.
The problems of plant spacing and plant support are considered in U.S. Pat. No. 3,927,491, issued to R. S. Farnsworth on Dec. 23, 1975. Farnsworth discloses a process for growing plants, such as lettuce, by supporting each plant on an individual "raft" floating on a nutrient solution. At the seedling stage, a group of small individual rafts may be connected together by frangible means. When the plants grow larger, the small rafts can be separated by breaking the frangible separators. The small rafts are then placed on larger rafts, which not only maintain a greater spacing between the plants but also are able to support the increased weight of the maturing plants. The rafts may be made of a buoyant material, such as polystyrene foam, cork, or the like.
As the plants continue to grow, they start to push apart the adjacent rafts. This allows light to reach the nutrient solution, an undesirable situation because it accelerates the growth of algae. It is also difficult to have a stable raft for each plant without making the rafts undesirably large, and individual rafts are not efficient from a materials handling standpoint.
Other patents have been issued relating to floating plant supports. For example, composite foam slabs floated on a nutrient solution in a large pan have been used for growing bean plants from seed, according to the disclosure in U.S. Pat. No. 3,798,836 of L. C. Rubens et al. Another method of hydroponic cultivation using buoyant plastic trays carrying grass plants on a nutrient stream flowing through a horizontal closed loop pipe is described in U.S. Pat. No. 3,579,907 of R. J. Graves. T. Kasahara et al. describe in U.S. Pat. No. 4,034,506, a porous buoyant plate consisting of foamed plastic beads bonded to each other, with water permeable spaces formed between the beads, for use as a hydroponic plant growing support. None of these latter patents is concerned, however, with changing the spacing of the plants as they grow, other than by traditional thinning.
The use of floating plant supports normally presents a problem with respect to supplying oxygen to the plant roots, so that some type of aeration system for the nutrient liquid is ordinarily required. On the other hand, both oxygen and liquid nutrient can be supplied readily to plants in non-floating supports. In U.S. Pat. No. 2,189,519 of M. W. Swaney, for example, plants are set in individual split-cork supports placed in spaced holes in the lid of a water-tight box or tank. Liquid nutrient partially fills the box, so that air can reach the roots in the space between the underside of the lid and the liquid surface. Unused holes in the lid may be plugged by solid corks to prevent light from entering the nutrient chamber. A different approach is followed by J. L. Jones in U.S. Pat. No. 4,075,785, which discloses a hydroponic growing system for lettuce in which individual plants are grown from seeds placed in individual growing blocks supported in fixed spaced relation in gravel beds, on A-frames, or by chicken wire stretched over inclined troughs. Liquid nutrient solution is periodically supplied to the plants by flooding or spraying. The plant roots are thus exposed to air during the intervals between nutrient applications.